1. Field of the Invention
This invention relates to a shift control system of an automatic transmission, and more particularly to improvements in a shift control system of an automatic transmission, wherein the shift control system includes at least a first and a second transmissions capable of automatically switching speeds separately of one another, and the first and the second transmissions are shifted simultaneously or alternately, to thereby achieve multi-speed shifts.
2. Description of the Prior Art
Along with a rapid spread in use of the automatic transmissions for motor vehicles in recent years, there have been commonly adopted such transmissions wherein a so-called over drive device, in which a transmission gear ratio is less than 1, is connected in series to the first transmission capable of automatic switching the shift speeds in association with a vehicle speed, a throttle opening, etc. as the second transmission.
Furthermore, there is also known such a transmission wherein, based on a function of the second transmission capable of switching from lower speed to higher speed and vice versa as the above-described over drive device, shift controls shown in FIG. 2A for example are performed, so that multi-speed shifts of six forward speeds can be achieved. This transmission is of such arrangement that a shift of the second transmission is actively cooperated with a shift of the first transmission, whereby the first transmission and the second transmission are shifted simultaneously or alternately, so that multi-speed shifts can be achieved.
The above-described arrangement makes it possible that the existing automatic transmission is utilized as the basis, and changes in design are minimized for manufacturing advantage, so that multi-speed shifts can be achieved. As the result, such advantages can be offered that the fuel consumption rate is improved, the power performance is bettered, and the burden of frictional materials is relieved due to making the speed shifts into multi-speed shifts, and the like.
However, in the automatic transmission wherein the first and the second transmissions are shifted simultaneously or alternately to achieve the multi-speed shifts, as shown in FIG. 2, there occurs a case where the first transmission is low gear shifted and the second transmission is high gear shifted, for example, like a shift from a third speed to a second speed and like a shift from a fifth speed to a fourth speed, to thereby down shift the automatic transmission as a whole. At this time, if only the respective shifts are controlled separately of one another, an increase in shift shock is not avoidable. Furthermore, for example, while a down shift is in operation, the shift is started from an up shift, or an up shift after a down shift is performed, thus presenting such a disadvantage that there may be experienced the shift characteristics of a strange driving feeling.